The present invention relates to a method for renewing the abrasive surface of grinding wheels. The method is particularly suitable for renewing the surface of wheels for grinding machines used in the ophthalmic field and comprising one or several diamond grinding wheels, gripping means designed to receive an ophthalmic lens, and control means. The invention also relates to a disk and a machine for carrying out the method.
Grinding machines are used in the ophthalmic field for trimming, bevelling or grooving ophthalmic lenses.
A trimming operation consists in adapting the generally circular contour of an ophthalmic lenses to the contour of the frame designed to receive it. It consequently consists in removing part of the material of the periphery of the lens.
Beveling of an ophthalmic lens, which is carried out after trimming, enables the cross-section of the peripheral edge of the lens to be adapted so that it fits into the groove which the spectacle frame usually incorporates. Beveling of the lens consists consequently in providing a generally triangular-shaped bevel or rib on the peripheral edge of the lens.
A grooving operation on an ophthalmic lens may also be carried out on a lens after it has been trimmed. This enables the cross-section of the peripheral edge of the lens to be adapted so that it can be mounted in certain types of frame. In effect, some frames do not include a groove over the whole internal perimeter of the parts designed to receive the lens; rather, it sometimes happens, for example, that the frame only matches the top part of the lens whereas the lower part of the lens is held by a transparent wire. In such cases, it is necessary to provide a groove or a channel which is most frequently triangular or semi-circular in shape, on the peripheral edge of the lens.
Most frequently, these three operations are carried out on one and the same grinding machine, fitted with a train of grinding wheels. Such machines are known and available commercially.
Grinding machines of this type typically comprise one or several diamond grinding wheels frequently coupled together to constitute a train of grinding wheels, gripping means designed to receive the ophthalmic lens to be machined, and control means. The control means are designed to control the rotation of the grinding wheel and the gripping means which hold the lens; the control means additionally control the relative displacement between the lens and the grinding wheel.
Such a grinding machine generally includes a sensing finger which is designed to follow a template corresponding to the particular shape of the lens. It may additionally, if appropriate, store the various shapes a lens may be desired to have in memory, and in this case, the sensing finger most frequently follows the contour of a disk.
Such a grinding machine generally operated in the following manner:
Initially a trimming wheel cuts out the lens to be machined after which a beveling or grooving wheel provides the bevel or groove in the cut-out lens.
Such machines and their operation are more particularly described in French patent number 2,543,039 in the name of the present applicant/assignee.
Known grinding machines and more particularly the one described in the said patent, operate in a highly satisfactory manner. They however do not provide a satisfactory solution to the problem of wear of the grinding wheel.
On such grinding machines, diamond grinding wheels are used. These generally consist of a disk, the periphery of which is covered with layer consisting of a binder incorporating fine diamond particles. The diamond layer can be produced by electro-erosion or by sintering. During repeated machining operation on lenses by the grinding machines, the grinding wheel surface becomes progressively more dull and loses its abrasive power. The loss is due to the diamond particles in contact with the lens to be machined gradually getting worn down. It is obvious that although the phenomenon is described for the case of diamond grinding wheels, it also occurs with other types of grinding wheel.
The wear of the grinding wheel may, under certain conditions, be compensated by a self-sharpening effect of the wheel on the glass to be machined. During machining, the ophthalmic lens being ground wears down the grinding wheel and thus partially renews the surface thereof. Nevertheless, such self-sharpening does not provide a satisfactory solution to the problem of wear of grinding wheels. Firstly, ophthalmic lenses are now constituted of very differing glasses having varying hardnesses which leads to poor self-renewal of the abrasive surface. In particular, certain glass-plastics composites may form a fine film on the wheel, the effect of which is to prevent or slow down self-renewal. Secondly, self-sharpening generally involves a deleterious modification to the shape of the wheel surface. Such self-sharpening does not in fact wear down the wheel uniformly and, if for example self-sharpening has occurred with a particular series of lenses, the resulting shape of the grinding wheel will doubtless have become modified to match the shape of the lenses. This consequently involves a risk of damaging lenses of a different shape during subsequent machining. This problem is obviously crucial in the case of grinding wheels for grooving and bevelling.
Manual renewal of grinding wheel surfaces has thus been proposed. When the operator considers that wheel is blunt, he employs a stick of alumina or carborundum or a material of similar hardness which he passes manually over the wheel surface to renew it. There are many disadvantages associated with this procedure:
Firstly, it leads to premature wear of the diamond surface when the resharpening operation are repeated frequently, or when the operator performs the operation too brutally. Even if the grinding wheel is still working, the operator may judge that machining is not proceeding fast enough and decide to renew the grinding wheel surface prematurely. This, bearing in mind the high cost of the grounding wheel, is a major disadvantage.
Secondly, the manual procedure, and despite all the experience the operator may possess, causes deformation of the periphery of the grinding wheel. In the case of a trimming wheel, with a flat peripheral cross-section, the alumina or carborundum stick must be held exactly parallel to the edge of the wheel to avoid deforming its profile. The operation is very difficult to carry out in a satisfactory way. In the case of a bevelling or grooving wheel which has a projecting or recessed profile, manual resharpening using an alumina or carborundum stick causes appreciable deformation of the machining profile of the grinding wheel. Because of this, after resharpening, the wheel no longer has a suitable shape to ensure completely satisfactory machining.
German Patent Application 4,012,658 discloses a system for truing a grinding wheel applied to a machine for grinding ophthalmic lenses. According to this document, the grinding wheel is trued using a tool that is mounted between the spindles in place of the lens. This system does not however enable the problem of grinding wheel surface renewal to be resolved. In all cases, the distance between the grinding wheel and tool axes stays practically constant and it is only possible to true the profile, but to not renew the grinding wheel surface. Moreover, the system has no provisions whatsoever for detecting when the grinding wheel needs truing.